1. Technical Field
This disclosure relates to a charging member used in an image forming apparatus of a copier, a laser beam printer and a facsimile or the like, also to a charging device including the charging member, a process cartridge including the charging device and an image forming apparatus including the process cartridge.
2. Description of the Related Art
A charging device that performs a charging processing against an image support body (photoconductive drum) is used in a conventional electro-photo type image forming apparatus of an electronic photo copier, a laser printer, a facsimile or the like. FIG. 6 is a schematic diagram of an electro-photo type image forming apparatus having a conventional charge roller.
In FIG. 6, 130 is a conventional electro-photo type image forming apparatus. The conventional electro-photo type image forming apparatus 130 includes a photoconductive drum 111 in which an electrostatic latent image is formed, a charging member (charge roller) 112 that performs the charging processing by contacting the photoconductive drum 111, an exposure device 113 of a laser beam or the like, a toner support body (image development roller) 114 that makes toner 115 to adhere onto the electrostatic latent image of the photoconductive drum 111, a transfer member (transfer roller) 116 that transfers a toner image on the photoconductive drum 111 onto a recording paper 117 and a cleaning member (cleaning blade) 118 for cleaning the photoconductive drum 111 after the transfer processing. In the FIG. 6, 119 are eliminated toners obtained from removing by the cleaning member 118 residual toners on a surface of the photoconductive drum 111, 120 is an image development device and 121 is a cleaning device. In addition, in the FIG. 6, functional units normally necessary at other electro-photo processes are not necessary in the present specification and are thereby abbreviated.
Next, basic image forming operations of the image forming apparatus 130 of the conventional electro-photo type are described.
When DC voltage is fed from a bias supply (not illustrated) against the charging roller 112 which is in contact with the photoconductive drum 111, the surface of the photoconductive drum 111 is charged uniformly to a high electric potential. Soon afterwards, when image light is projected by the exposure device 113 onto the surface of the photoconductive drum 111, an electric potential of a part irradiated by the photoconductive drum 111 decreases. Such a charging mechanism by the charging roller 112 towards the surface of the photoconductive drum 111 is in fact known to be discharges according to the Paschen's Law in an infinitesimal space between the charging roller 112 and the photoconductive drum 111.
Image light is light quantity distributions according to black/white of an image. When such image light is projected, due to irradiations of the image light, an electrical potential distribution corresponding to a recorded image, that is, an electrostatic latent image is formed on a surface of the photoconductive drum 111. In such a way, when the part of the photoconductive drum 111 formed with the electrostatic latent image passes through the image development roller 114, toners become adhered in correspondence to highs or lows of the electrical potential and a toner image visualizing the electrostatic latent image is formed. The recording paper 117 is carried at a predetermined timing by resist rollers (not illustrated) to the part of the photoconductive drum 111 formed with the toner image to double the toner image. Then after the toner image is transferred onto the recording paper by the transfer roller 116, the recording paper 117 becomes separated from the photoconductive drum 111. The separated recording paper 117 is carried through a transport path. The recording paper 117 is then heated and fixed by a fixing unit (not illustrated) to be ejected outside the machine. When transfer is terminated in such a way, the surface of the photoconductive drum 111 is applied cleaning processing by the cleaning member 118. Furthermore, residual electric charges are removed by a quenching lamp (not illustrated) to prepare for the next image forming process.
With regard to a charging member used in such an image forming apparatus, a charging by contact type is widely used in which the charging member contacts the surface of the image support body so that the surface of the image support body is charged. During usage of the charging member, adherent substances such as residual toners on the surface of the image support body, toners deteriorated due to oxidized gas generated by discharges of the charging member and toner constituent substances or the like adhere to the surface of the charging member so that taint generated as such to the charging member becomes problematic. Therefore, in order to remove these adherent substances, the cleaning member is disposed on the surface of the charging member. However, the cleaning member becomes tainted over time by the adherent substances removed from the charging member by the cleaning member so that cleaning capabilities of the cleaning member decreases. Therefore, discharge irregularities are generated by the adherent substances adhered to the charging member and abnormal images generated become problematic.
Thereby a non-charging by contact type charging member is gradually adopted to charge the image support body surface in which the charging member is disposed to be in close contact with the image support body with an constant space (minute gap) maintained between the charging member and the image support body. In the case the charging member of the non-charging by contact type is used, the charging member is not in direct contact with the image support body so that the surface of the charging member becomes difficult to be tainted. Therefore, the charging member of the non-charging by contact type is possible to have a longer life span than the charging member of the charging by contact type. However, in the case the charging member of the non-charging by contact type is used, in order to prevent charge irregularities due to changes of the minute gap, high voltage AC voltage is impressed doubly with DC voltage to the charging member. Therefore, adherents of the image support body flies between the image support body and the charging member so that over time, adherents of the image support body is gradually accumulated on the surface of the charging member. Therefore, finally, irregularities of electrical resistances are generated due to the adherents adhered to the surface of the charging member. Discharge irregularities are generated due to the irregularities of the electrical resistances so that it is problematic that abnormal images are generated.
By the way, if the minute gap between the image support body and the charging member can be widened as much as possible, then the adherence of tainted substances can be reduced. But if the minute gap becomes too wide, voltage of the AC voltage needs to be heightened. Therefore, abnormal discharges due to leakage become easy to be generated so that abnormal images (white spot) are easily generated. Therefore, the minute gap between the image support body and the charging member can not be widened.
In addition, the charging member used for the image forming apparatus is necessarily electrically conductive. In order to realize the electrically conductive charging member, if for example, carbon black of an electrically conductive system is used as a conductive agent, abnormal discharges due to local leaks become easy to be generated when high AC voltage is impressed. Therefore, minute gaps between the image support body and the charging member can not be widened. Therefore, a conductive agent of an ion conductive system is preferable for the conductive agent that makes the charging member electrically conductive. But lower electrical resistance of a coating material is difficult to be realized if a conventional conductive agent of the ion conductive system is used. Therefore, with reference to Laid-open Japanese patent publication No. Hei 7-77859, conventionally, in order to reduce leaks of the charging member, an electrical resistance adjustment layer that constitutes the charging member is formed using an adjustment layer of the ion conductive system whereas a surface layer that constitutes the charging member is formed using a surface layer of the electrically conductive system. However, such constitutions are not sufficient.